The present invention relates generally to testing devices and methods and, more particularly, to a testing method and apparatus for detecting and locating faults in laminated cores of electric machines.
For the purposes of the following discussion, a lamination segment of the stator of a large generator may be considered as an example structure on which the present method and apparatus is suited for performing lamination fault testing. A conventional laminated core segment of a large generator stator includes a back iron portion, teeth and slots. Lamination segments are typically formed into a magnetic core by stacking. A plurality of lamination segments (eighteen lamination segments each being twenty degrees, as one example) may be used to form a complete first lamination layer with the next plurality of lamination segments forming a complete second lamination layer on top of and offset from the lamination segments in the first lamination layer. Such stacking continues until formation of a short stack of about 2.54 centimeters to about 10.16 centimeters thick. A plurality of short stacks are further joined and/or clamped by bolts and/or other mechanical devices to form a stator core. A typical large generator stator core may have a diameter, for example, ranging from about one meter to about three meters and a length ranging from about one meter to about ten meters.
Lamination faults in a stator core, such as short circuited laminations, may become highly destructive in large electric machines. Inter-lamination short circuits (caused by mis-operation or manufacturing defects such as burrs, defects in lamination coating, damage during assembly) cause eddy currents to flow through the shorted laminations and key bars. These currents are driven by the time varying flux in the stator core present during normal generator operation. The heating caused by these currents can cause burning and melting of the laminations at the location of the defect. The additional heating can also cause insulation degradation and failure in the stator bars. If these defects are found during the manufacturing or rewinding operations, they can be corrected. Consequently, it is desirable to have an easy and efficient method and apparatus for testing laminated stator cores for such faults accurately, within as fine a resolution as reasonably feasible. Moreover, it is also desirable to have a stator core testing apparatus and method that is easily implemented both during the manufacture of the core and during routine maintenance or service procedures of the electric machines in which such laminated cores are used.
One well known conventional stator core testing method, more commonly known as a “ring test”, employs a technique of exciting the stator laminations at a rated operating induction level. The ring test relies upon the detection of eddy current heating caused by short circuit currents in the laminations. The generator stator core is specially wound with an excitation winding having a number of turns of cable in the manner of a toroid. The current level in the windings is chosen such that the flux driven in the core is near normal operating levels. Local temperature differences produced by eddy currents due to an interlamination short can be detected by an infrared scanner. Unfortunately, the ring test requires the use of a controllable high-power, high-voltage source and special stator core excitation windings with large cross sections. Short circuits that are located below the surface of the stator teeth and slots are difficult to find, since thermal diffusion causes the surface temperature rise to become diffuse. Moreover, because of the high power levels used in the ring test, personnel are not allowed in the bore of the stator core during testing. In addition, cables used in the test must be appropriately sized to accommodate the high power level which inevitably leads to long setup and removal times. These drawbacks and the high power requirements cause this method to be usually impractical for field test applications.
Another known inspection technique, such as disclosed in UK Patent 2,044,936 to Sutton, involves detecting changes in the flux fields due to interlamination shorts with weak induction. This technique is commonly referred to as an Electromagnetic Core Imperfection Detector (EL CID) test. With this test, a core stack is magnetized at a much lower magnetic flux level as compared to its rated operating level and, consequently, only a low power, low-voltage power supply is needed. Each tooth-pair is then scanned with a special detector coil system to look for anomalies in the flux. As in the ring test, a disadvantage of this testing method is that it also requires a special winding for the stator core.
In yet another approach, as described in commonly assigned U.S. Pat. Nos. 6,469,504 and 6,489,781 both to Kliman et al., stator core lamination faults are more easily and efficiently detected through the use of a flux-injection testing probe of the type, for example, as depicted in FIG. 1. This flux-injection probe testing approach, as described in the above mentioned applications, has discernable advantages as compared to prior embodiments. For example, short stacks of laminations may be tested individually while stacking during core fabrication and/or during core servicing so that, if a fault is detected, remedial measures may be performed on the affected lamination immediately rather than having to substantially disassemble a completed core to access a fault later determined to be located in the middle of the core.
However, when the thickness of a flux-injection type probe exceeds the thickness of about two or three core laminations, sensitivity and selectivity are reduced. The sensor magnetic yoke itself will influence the losses measured along the core. Unfortunately, physical and practical constraints limit the minimum feasible sensor yoke thickness. Moreover, magnetic flux sensitivity is also influenced by the distance between the magnetic yoke and the core laminations. Typically, about 50-75 micrometers of lamination stagger may result from punching tolerances and assembly variability when fabricating laminated cores. In electric machines that function as generators, lamination core stagger is typically filled in and covered up by layers of thick paint. Such paint further increases the effective gap between the magnetic yoke and the core which correspondingly reduces the effective sensitivity of the testing probe. In addition, incremental core losses due to individual lamination faults can often be quite small e.g., on the order of 1% or less. For at least the above reasons, it is very difficult to detect some small faults and, especially, small lamination faults when using known flux-injection probe devices and methods. Consequently, it would be highly desirable to have a core fault detection method and apparatus that provides a significant increase in sensitivity over the prior known art.